1. Field of the Invention
The invention relates to the field of light modulating devices, especially to light modulating devices used in holographic displays.
2. Technical Background
Computer-generated video holograms (CGHs) are encoded in one or more spatial light modulators (SLMs); the SLMs may include electrically or optically controllable cells. The cells modulate the amplitude and/or phase of light by encoding hologram values corresponding to a video-hologram. The CGH may be calculated e.g. by coherent ray tracing, by simulating the interference between light reflected by the scene and a reference wave, or by Fourier or Fresnel transforms; CGH calculation methods are described for example in US2006/055994 and in US2006/139710, which are incorporated by reference. An ideal SLM would be capable of representing arbitrary complex-valued numbers, i.e. of separately controlling the amplitude and the phase of an incoming light wave. However, a typical SLM controls only one property, either amplitude or phase, with the undesirable side effect of also affecting the other property. There are different ways to spatially modulate the light in amplitude or phase, e.g. electrically addressed liquid crystal SLM, optically addressed liquid crystal SLM, magneto-optical SLM, micro mirror devices or acousto-optic modulators. The modulation of the light may be spatially continuous or composed of individually addressable cells, one-dimensionally or two-dimensionally arranged, binary, multi-level or continuous.
In the present document, the term “encoding” denotes the way in which regions of a spatial light modulator are supplied with control values to encode a hologram so that a 3D-scene can be reconstructed from the SLM.
In contrast to purely auto-stereoscopic displays, with video holograms an observer sees an optical reconstruction of a light wave front of a three-dimensional scene. The 3D-scene is reconstructed in a space that stretches between the eyes of an observer and the spatial light modulator (SLM), or possibly even behind the SLM. The SLM can also be encoded with video holograms such that the observer sees objects of a reconstructed three-dimensional scene in front of the SLM and other objects on or behind the SLM.
The cells of the spatial light modulator may be transmissive cells which are passed through by light, the rays of which are capable of generating interference at least at a defined position and over a spatial coherence length of a few millimeters. This allows holographic reconstruction with an adequate resolution in at least one dimension. This kind of light will be referred to as ‘sufficiently coherent light’.
In order to ensure sufficient temporal coherence, the spectrum of the light emitted by the light source must be limited to an adequately narrow wavelength range, i.e. it must be near-monochromatic. The spectral bandwidth of high-brightness LEDs is sufficiently narrow to ensure temporal coherence for holographic reconstruction. The diffraction angle at the SLM is proportional to the wavelength, which means that only a monochromatic source will lead to a sharp reconstruction of object points. A broadened spectrum will lead to broadened object points and smeared object reconstructions. The spectrum of a laser source can be regarded as monochromatic. The spectral line width of a LED is sufficiently narrow to facilitate good reconstructions.
Spatial coherence relates to the lateral extent of the light source. Conventional light sources, like LEDs or Cold Cathode Fluorescent Lamps (CCFLs), can also meet these requirements if they radiate light through an adequately narrow aperture. Light from a laser source can be regarded as emanating from a point source within diffraction limits and, depending on the modal purity, leads to a sharp reconstruction of the object, i.e. each object point is reconstructed as a point within diffraction limits.
Light from a spatially incoherent source is laterally extended and causes a smearing of the reconstructed object. The amount of smearing is given by the broadened size of an object point reconstructed at a given position. In order to use a spatially incoherent source for hologram reconstruction, a trade-off has to be found between brightness and limiting the lateral extent of the source with an aperture. The smaller the light source, the better is its spatial coherence.
A line light source can be considered to be a point light source if seen from a right angle to its longitudinal extension. Light waves can thus propagate coherently in that direction, but incoherently in all other directions.
In general, a hologram reconstructs a scene holographically by coherent superposition of waves in the horizontal and the vertical directions. Such a video hologram is called a full-parallax hologram. The reconstructed object can be viewed with motion parallax in the horizontal and the vertical directions, like a real object. However, a large viewing angle requires high resolution in both the horizontal and the vertical direction of the SLM.
Often, the requirements on the SLM are lessened by restriction to a horizontal-parallax-only (HPO) hologram. The holographic reconstruction takes place only in the horizontal direction, whereas there is no holographic reconstruction in the vertical direction. This results in a reconstructed object with horizontal motion parallax. The perspective view does not change upon vertical motion. A HPO hologram requires less resolution of the SLM in the vertical direction than a full-parallax hologram. A vertical-parallax-only (VPO) hologram is also possible but uncommon. The holographic reconstruction occurs only in the vertical direction and results in a reconstructed object with vertical motion parallax. There is no motion parallax in the horizontal direction. The different perspective views for the left eye and right eye have to be created separately.
A computer-generated hologram may be represented as an array of complex numbers. A device for reconstruction of such a hologram has to include one component which is a medium for displaying the hologram data. Writing the data onto the medium may be done either once as in the case of a fixed holographic optical element, for example in lithographic structures, or as a function of time as in the case of addressable structures, which allow one to display time-varying content.
In this document the term ‘pixelated optical element’ or ‘diffractive element’ is used for a medium with fixed content; the term ‘spatial light modulator’ (SLM) is used for a medium with addressable time-variable content, which may be re-written as a function of time. In a more general manner what is described in this document by means of hologram data also holds for other tasks where either a fixed or a variable medium can be used for some kind of light modulation. In this document the term light modulation element is used for a fixed element, or for a variable element or for a combination of both types of element.
Light modulating elements may be either transmissive or reflective. In this document the term transmission may be used—in a more general manner—such that it also refers to reflection in the case of a reflective display or as an interaction between the optical element and the light.
SLM or diffractive elements may be either transmissive or reflective. In this document the term transmission may be used in a more general manner—such that it also refers to reflection in the case of a reflective display or as an interaction between the SLM or diffractive elements and the light.
There exist SLMs (i.e. variable light modulators) with a fixed intrinsic pixel structure and other types of SLM where this does not hold: for example, optically addressable SLMs. Where the following description refers to a pixelated SLM it also includes such types of SLM which do not have an intrinsic pixel structure, but on which some kind of grid pattern similar to a pixel structure can be achieved by the writing process.
For writing of holographic data, many combinations of SLMs and diffractive elements may be used, ranging from a single SLM and a single diffractive element, up to a combination of several SLM and several diffractive elements, any given combination being able to display complex numbers. However it is also possible that each single complex number of an array of hologram data may be represented by a single pixel or by a group of usually adjacent amplitude and/or phase pixels in either an SLM or in a diffractive element.
Each pixel of the SLM/diffractive element usually is able to display only a limited number of different values. For these values the term “quantization steps” is used. For example a common amplitude SLM has 256 quantization steps.
When writing the hologram data onto the SLM/diffractive element a quantization of the hologram data is necessary. For example a rounding of hologram data values to the quantization steps of the SLM/diffractive element should take place. For a hologram, this quantization may result in deviations from the desired hologram reconstruction. These errors may be small and tolerable in the case of a large number of quantization steps but they become more significant and may be not tolerable in the case where only a small number of quantization steps exist. The number of quantization steps needed may vary depending on other parameters of the application.
Some types of SLM are binary which means they have only 2 quantization steps i.e. they have only 0 (zero) and 1 (one) states. Examples are ferroelectric liquid crystal (FLC) SLMs or micromirror arrays. There exist also other types of SLM with more than 2 but still relatively few quantization steps, for example ternary SLMs with 3 quantization steps.
FLC SLM may be configured either as amplitude or as phase SLMs. A configuration suitable for use as phase SLMs is described in G. D. Love, and R. Bandari, Optics Communications, Vol. 110, 475-478, (1994). Also micromirror arrays may be configured either as amplitude SLMs—for example by use of micromirror tilt, or as phase SLMs—for example by use of micromirror pistons.
SLMs with only a few quantization steps may have advantages, for example fast switching times which allow high frame rates, which make their use desirable.
3. Discussion of Related Art
WO 2004/044659 (US2006/0055994) filed by the applicant and incorporated by reference describes a device for reconstructing three-dimensional scenes by way of diffraction of sufficiently coherent light; the device includes a point light source or line light source, a lens for focusing the light and a spatial light modulator. In contrast to conventional holographic displays, the SLM in transmission mode reconstructs a 3D-scene in at least one ‘virtual observer window’ (see Appendix I and II for a discussion of this term and the related technology). Each virtual observer window is situated near the observer's eyes and is restricted in size so that the virtual observer windows are situated in a single diffraction order, so that each eye sees the complete reconstruction of the three-dimensional scene in a frustum-shaped reconstruction space, which stretches between the SLM surface and the virtual observer window. To allow a holographic reconstruction free of disturbance, the virtual observer window size must not exceed the periodicity interval of one diffraction order of the reconstruction. However, it must be at least large enough to enable a viewer to see the entire reconstruction of the 3D-scene through the window(s). The other eye can see through the same virtual observer window, or is assigned a second virtual observer window, which is accordingly created by a second light source. Here, a visibility region i.e. the range of positions from which an observer can see a correct reconstruction, which would be rather large, is limited to the locally positioned virtual observer windows. This virtual observer window solution uses the larger area and high resolution of a conventional SLM surface to generate a reconstruction which is viewed from a smaller area which is the size of the virtual observer windows. This leads to the effect that the diffraction angles, which are small due to geometrical reasons, and the resolution of current generation SLMs, are sufficient to achieve a high-quality real-time holographic reconstruction using reasonable, consumer level computing equipment.
In WO 2004/044659 (US2006/0055994) and in some other patent applications of the applicant (e.g. WO 2006/066919, WO 2006/027228 or WO 2006/066906), a method and device for reconstructing holograms is described, where a reconstruction of a three dimensional (3D) scene can be seen from within a virtual observer window. The observer window may have approximately the size of one eye. One example of such a device—suitable for more than one observer—includes a time sequential generation of the observer windows for each observer as well as for the right and the left eye of each observer. For such an implementation it would be desirable to use as one element of the device a fast switching SLM.
In general there may be other types of holograms and holographic displays, different from the type described in WO 2004/044659 (US2006/0055994) for which fast switching SLMs are also advantageous.
In standard (i.e. non holographic) use as amplitude displays, binary SLM make use of a method called ‘pulse width modulation’ where grey values are emulated by time average over several on and off cycles of binary states. This method is usually not applicable for holographic use, because modulation of coherent light—needed for a hologram reconstruction—can only be obtained from those hologram data displayed at the same time.
Diffractive elements may also exist in a binary form, or with a larger number of quantization steps. For example, state of the art phase elements can be manufactured with 64 quantization steps or even more. For amplitude diffractive elements there is the possibility to make use of grey scale lithography in order to obtain non-binary elements. Also there exist special glass materials through which transmission can be varied continuously.
For reducing quantization errors in binary diffractive elements there exist iterative calculation methods. But these require high calculation effort and for this reason and other reasons such calculation methods may not be suitable for fast calculation of variable hologram content to be displayed with an SLM.
For binary amplitude SLMs or binary diffractive amplitude elements it is known that several adjacent pixels may be combined to form a macropixel in order to emulate grey levels. By switching on a different number of binary pixels the total transmittance of the macropixel is changed. This works similarly to half tone printing. A disadvantage of this method is the fact that with a macropixel composed of N individual binary pixels it is only possible to obtain N+1 grey values.
In the patent application US20070109617 a combination of a pixelated SLM with a pixelated phase mask diffractive element is described, where the phase mask has a higher resolution (i.e. a smaller pixel size) compared to the SLM. Each pixel of the SLM is in its effect on hologram reconstruction combined with several pixels of the phase mask. The aim of US20070109617 is to increase the useable diffraction angle. But this leads to the disadvantage of a higher noise level.